2002 str brake pedal drops

[RobXJC]

Afternoon all

Bit of an odd issue with the brake pedal on my 2002 str. Basically start the engine and brake pedal drops about 10mm which is enough to allow the brake lights to turn on. With the engine off I can push the brake pedal two of 3 times to bring back to its correct position to turn the brake lights off. Start the engine and brake pedal drops about 10mm again.
There are no abs or traction control warnings on the dash and no engine management light on either.

Any ideas anyone?

[RobXJC]

Update.
Needed a replacement brake booster/servo.
The valve between the front and back of the servo diaphragm had blocked which didn’t balance the vacuum so created vacuum on the master cylinder side only which in turn when the engine was running would cause the brake pedal to drop and apply the brakes.
For a really cheap replacement servo look at ate Volvo XC90 parts. The pedal attachment is correct, the brake pedal switch and cruise switch are also correct. The mod is the vacuum attachment. The Jag version is to the left of the master cylinder, the Volvo version is to the right of the master cylinder. Found this out after buying a Jag version at £150. Volvo version £30. Both used parts in similar condition for sale on the fleabay
Hope this info helps someone in the future

[andypryce]

Good update.